Add WordRepository integration tests with embedded MongoDB

[Copilot]

• Add EphemeralMongo7 runtime packages
• Create MongoDbTestRunner.cs
• Create WordRepositoryTests.cs (48 integration tests)
• Add [Category("IntegrationTest")] and test:integration / test:unit scripts
• Update WordRepositoryTests.cs to match updated WordRepository:
  • Renamed RepoCreate → Create
  • Renamed RepoUpdateFrontier → UpdateFrontier
  • Renamed RepoReplaceFrontier → ReplaceFrontier
  • Renamed RepoRevertReplaceFrontier → RevertReplaceFrontier (returns bool)
  • Renamed RepoDeleteFrontier → DeleteFrontier
  • Renamed RepoRestoreFrontier → RestoreFrontier (single string arg, returns bool)
  • Reordered test sections to match WordRepository method order
  • Fixed assertions for changed return types

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This change is 

[imnasnainaec]

@copilot Add an "IntegrationTest" category for all these new tests. Add test:integration and test:unit scripts in package.json to test all in and out of that catagory.

[Copilot]

@copilot Add an "IntegrationTest" category for all these new tests. Add test:integration and test:unit scripts in package.json to test all in and out of that catagory.

Done in commit 043333f.

• [Category("IntegrationTest")] added to the WordRepositoryTests class (applies to all 48 tests)
• test:integration runs dotnet test --filter "TestCategory=IntegrationTest" (48 tests)
• test:unit runs dotnet test --filter "TestCategory!=IntegrationTest" (all 1101 non-integration backend tests)

[imnasnainaec]

@copilot I've rebased this on an updated mt2 branch. Update the tests to match the new WordRepository, including the order of the methods.

[Copilot]

@copilot I've rebased this on an updated mt2 branch. Update the tests to match the new WordRepository, including the order of the methods.

Done in commit 9adeb0f. Updated the tests to match the new WordRepository:

• RepoCreate → Create
• RepoUpdateFrontier → UpdateFrontier
• RepoReplaceFrontier → ReplaceFrontier
• RepoRevertReplaceFrontier → RevertReplaceFrontier (now returns bool; fixed assertions and renamed EmptyListsReturnsEmpty → EmptyListsReturnsTrue)
• RepoDeleteFrontier → DeleteFrontier
• RepoRestoreFrontier → RestoreFrontier (now takes a single string and returns bool; replaced the empty-list test with TestRestoreFrontierNotFoundReturnsFalse)
• Test sections reordered to match WordRepository method order

[imnasnainaec]

@copilot I've rebased this on an updated mt2 branch. Update the tests to match the new WordRepository.

[Copilot]

@copilot I've rebased this on an updated mt2 branch. Update the tests to match the new WordRepository.

Done in commit a27113e. The rebase added an explicit ArgumentException check in RestoreFrontierWithSession when the word's ID is already in the Frontier (previously MongoDB would throw a MongoWriteException on the duplicate key violation). Added TestRestoreFrontierAlreadyInFrontierThrows to cover this new behavior.

[imnasnainaec]

Subsumed in #4198